This invention relates to an improved process for the production of sugars and anhydrosugars from biomass materials such as wood or other lignocellulosics, or from biomass derived materials as newsprint or other paper goods, wood pulps, waste fibre etc.
Scott & Pisorz U.S. Pat. No. 4,880,473 discloses a process for preparation of solutions of fermentable sugars using a fast pyrolysis of pretreated biomass. In that invention, the pretreatment was a dilute acid prehydrolysis which removed the majority of the hemicellulose fraction as well as the alkali and alkaline earth cations associated with the biomass. As a result, as much as 80% of the lignin appeared in the pyrolysis liquid as pyrolytic lignin, i.e. the volatile decomposition products primarily derived from the lignin fraction of biomass. A portion of these lignin-derived compounds can be separated by water extraction from the more soluble sugars and anhydrosugars derived from the cellulose fraction, and which are recovered in the aqueous phase.
It is well known that the alkaline cations associated with the biomass act to promote certain thermal decomposition reactions, in particular those reactions which lead to char or gas production. These cations are known to be able to be removed by treatment with very dilute acid at normal temperatures, such that the lignin, hemicellulose and cellulose fractions of the biomass are substantially unaffected. The resulting deionized biomass differs from that given a prehydrolysis pretreatment in that the hemicellulose content is retained in the solid pretreated lignocellulose.
The behaviour of deionized wood has been shown in laboratory batch pyrolysis reactions carried out at very small scale under vacuum with slow heating to be different from that of normal wood. The removal of the cations enhanced the yield of anhydrosugars derived from cellulose, and particularly increased the yield of anhydroglucose, (levoglucosan). Typically, deionization was carried out by batch-wise contacting of the wood with very dilute acid, for example 0.1% sulfuric or hydrochloric acid, and then washing it thoroughly with distilled or deionized water to remove the reagent. The deionized wood was then pyrolyzed under vacuum in microscale laboratory batch reactors in the absence of air or oxygen. Much poorer yields of levoglucosan were obtained from these batch reactions when carried out at atmospheric pressure. Sugars or anhydrosugars derived from the pyrolysis of the hemicellulose fraction have apparently not previously been reported as products obtained from fast pyrolysis of deionized lignocellulosics. This suggests that at the laboratory conditions used in earlier work of others, thermal fragmentation of the hemicelluloses predominated over depolymerization reactions.
Other methods of converting cellulose to fermentable sugars, mainly glucose, are carried out in the liquid phase and require digestion in acid. If this digestion is done using a strong acid, many undesirable and toxic side products are formed. If weak acid is used, poorer glucose yields and more dilute solutions are obtained. In this context, it is also known to use a weak acid treatment to prehydrolyze cellulosic raw material followed by enzymatic hydrolysis. This procedure can yield an improved product at higher concentrations compared to weak acid hydrolysis above. However this process is relatively complex and expensive. Other liquid phase methods such as those known as steamexplosion, may also be cited, for example in Hinger, U.S. Pat. 4,468,256 where glucose is produced from vegetable raw material by impregnating the raw material with dilute acid and subsequent heating to a temperature of above 250.degree. C. by means of high pressure steam. This heating step takes place in a short time accompanied by a sudden pressure rise. In these liquid phase acid digestion methods the principal product from the cellulose is glucose, and anhydrosugars such as levoglucosan are largely absent.
A pyrolysis process for the production of levoglucosan from wood has been described in Esterer U.S. Pat. No. 3,309,356 and in Peniston U.S. Pat. No. 3,374,222. These processes did not use any pretreatment of the wood but stated that it was unnecessary. Also, the pyrolysis process used gave a two-phase liquid. Further, the relatively long residence time of the condensable volatiles in the reactor, approximately 15 seconds, would reduce significantly the possible yields obtainable. In the above Esterer patent it was necessary to remove the lignin-derived material soluble in the aqueous phase by a solvent extraction procedure. In the Peniston patent, these materials as well as carbohydrate derived acids were removed by conversion to insoluble metal salts.
The oxidation of biomass materials in air or oxygen has been very extensively studied in combustion and gasification processes. In particular, the effect of added cations or of the indigenous cations of the wood on the combustion behaviour has been widely reported in connection with fire studies. The indigenous alkaline cations such as K, Ca, Na, Mg or additions of these cations are well known to be effective gasification catalysts in air or oxygen blown systems. Fast pyrolysis with added air to produce liquids and char from biomass has been reported as part of the LEBEN program in Italy, carried out under the sponsorship of the European Economic Community. In general, use of air or oxygen in the fast pyrolysis of biomass results in reduced liquid yields (20-40% vs. 50-80% for non-oxidative pyrolysis) and in degradation of the product in terms of yields of specific desirable chemicals.
There is a need for a more economic process for the deionization of wood or other biomass or biomass-derived materials for the purpose of carrying out a fast pyrolysis process for the conversion by thermal means of cellulose or hemicellulose to sugars and anhydrosugars, and which would allow these processes to become competitive with other processes for the conversion of cellulose or hemicellulose to anhydrosugars or fermentable sugars. Improvements in the fast pyrolysis process would also be valuable in reducing the cost of this method of conversion of hemicellulose or cellulose to sugars or anhydrosugars.